1. Field of the Invention
This invention relates to a hydraulic control system of an automatic transmission for an automotive vehicle and more, particularly to a hydraulic control system of automatic transmission wherein line pressure is variably controlled to improve the power train efficiency and the response of shifting time during gear shifting.
2. Description of the Conventional Art
A conventional vehicle automatic transmission has a torque converter and a multiple stage transmission gear mechanism connected with the torque converter, which includes hydraulically actuated friction members for selecting one of the gear stages of the transmission gear mechanism in accordance with vehicle operating conditions.
The hydraulic control system pressurized by a hydraulic pump provides the hydraulic pressure required to operate the friction members and control valves.
A commonly used automatic transmission of a vehicle has a hydraulic torque converter which generally includes a pump impeller connected with an engine output shaft to be driven thereby, a turbine runner with an output shaft member, and a stator disposed between the pump impeller and the turbine runner, so that the fluid is circulated by the engine driven pump impeller through the turbine runner with the aid of the stator which functions to deflect the fluid from the turbine runner to a direction where the fluid flow does not disturb the rotation of the pump impeller when the fluid flows into the pump impeller.
The automatic shift is made by operating the friction members such as clutches or a kick down brake at each shift change. Also, a manual valve of which ports are converted by selecting a position of a selector lever, is designed to be supplied with fluid from a hydraulic pump and to supply the fluid to a shift control valve. In a 4-speed automatic transmission, the shift control valve has an operated port made by an electronic control system.
An example of a hydraulic pressure control system of an automatic transmission for a vehicle will be described with reference to FIG. 8 which shows a conventional hydraulic control system comprising a torque converter 1 attached to the engine through the flexible engine plate, and rotating at engine speed for transmitting power of the engine to an input shaft of the transmission gear mechanism, a damper clutch control valve 2 for controlling the application and release of a damper clutch to increase the power train efficiency inside the torque converter 1, a regulator valve 5 for regulating the output hydraulic pressure of the hydraulic pump 4 according to the automatic transmission requirements, and a reducing valve 6 for stably regulating the supply of the hydraulic pressure to a solenoid valve and the damper clutch control valve 2.
A manual valve 7, which is connected to an outlet of the hydraulic pump 4 and provided with the hydraulic pressure, is designed to deliver the line pressure to the regulator valve 5 and a shift control valve. In the manual valve 7, lands are changed according to the position of a shift selector lever.
A shift control valve 8, which is operated in response to two shift control solenoid valves A and B controlled by a TCU (Transmission Control Unit), is designed to selectively transmit the hydraulic pressure through a first-second speed shift valve 9, an end clutch valve 10, a second-third and third-fourth speed shift valve 11, and a rear clutch exhaust valve 12 to a front clutch 13, a rear clutch 14, a low-reverse brake 15, a kick down servo brake 16, an end clutch 17, and the like. An N-D control valve 18 is connected to the rear clutch 14. An N-R control valve 19 is connected to the first-second speed shift valve so as to reduce the impact caused by the shift.
Also, a pressure control solenoid valve 20 is connected to a pressure control valve 21 to reduce the shock produced by the control at the time of shifting.
According to the location of a valve spool within the shift control valve 8, which is controlled by the shift control solenoid valves A and B, the fluid pressurized in the hydraulic pump 4 is supplied through a first conduit D1, a second conduit D2, a third conduit D3, and a fourth conduit D4. When the shift select lever of the manual valve 7 is at an "R" range, the pressurized fluid is supplied through of a reverse conduit R1. Following is the description a shifting operation in the system of FIG. 8.
When the shift select lever is at a "D" range, the fluid pressurized in the hydraulic pump 4 is supplied through a conduit L1 into the manual valve 7, which is further supplied through a conduit L2 into the shift control valve 8 and the first conduit D1.
At a first speed of the "D" range, both of the shift control solenoid valves A and B are controlled to an ON state by the TCU, and therefore the hydraulic pressure passing through the shift control valve 8 is exhausted to effect no change in the location of the valve spool. At the same moment, the TCU controls the pressure control solenoid valve 20 to an ON state, and a part of the pressurized fluid returning through the reducing valve 6 is exhausted.
However, because the hydraulic pressure is not supplied into the first-second speed shift valve 9, the hydraulic pressure passing through the conduit D1 is supplied via the rear clutch exhaust valve 12 into the rear clutch 14 to actuate the same.
At a second speed of the "D" range, the shift control solenoid valve A is controlled to an OFF state by the TCU, and the hydraulic pressure is exhausted toward the shift control solenoid valve B to displace the valve spool and a plug of the shift control valve 8 rightward in FIG. 8 so that the hydraulic pressure flowing into the manual valve 7 is supplied through the conduit D2.
Accordingly, the hydraulic pressure passing through the conduit D2 is supplied to the left side of the first-second speed shift valve 9 to push the valve spool rightward in order to provide for a third speed. At this moment, the pressure control solenoid valve 20 is controlled to an OFF state to keep the hydraulic pressure from being exhausted, and thus the fluid pressurized in the hydraulic pump 4 is supplied to the left side of the pressure control valve 21 via the reducing valve 6 and a hydraulic conduit L3 to push the valve plug therein rightward. Accordingly, the hydraulic pressure passing the first conduit D1 returns to the first-second speed shift valve 9 via the N-D control valve 18.
Because the valve spool of the first-second speed shift valve 9 is pushed rightward, the hydraulic pressure which has passed the N-D control valve 18 is supplied to the kick down servo brake 16 to actuate the same, and the second speed is accomplished thereby.
At a third speed of the "D" range, because both of the shift control solenoid valves A and B are controlled to the OFF state by the TCU, the hydraulic pressure is kept from being exhausted, the valve spool of the shift control valve 8 is moved further rightward, and the valve plug remains stopped by means of a stopper.
At this state, because the second and third conduits D2 and D3 are opened simultaneously, the hydraulic pressure passing through the second conduit D3 and coming into the right side of the end clutch valve 10 pushes the valve plug leftward and enters the end clutch 17 to actuate the same.
The pressurized fluid is controlled by passing through the first-second speed shift valve 9 via the pressure control valve 21 and is passed through the second-third and fourth-third speed shift valve 11. Then, a part of the pressurized fluid acts to release the kick down servo brake 16 and another part of the pressurized fluid acts to actuate the front clutch 13,
At this moment, the servo brake 16 which was actuated at the second speed is deactivated by the action of the hydraulic pressure supplied through the conduit connected to the front clutch 13.
At a fourth speed of the "D" range, because only the shift control solenoid valve B is controlled to the OFF state by the TCU, the valve spool of the shift control valve 8 is moved father rightward than it is at the third speed to open the fourth conduit D4.
Then, the hydraulic pressure is supplied to the left side of the rear clutch exhaust valve 12 to push the valve spool therein rightward to stop supplying the hydraulic pressure for actuating the front clutch 13 and the hydraulic pressure for deactivating the kick down servo brake 16. Accordingly, the kick down servo brake 16 is actuated again automatically, the end clutch 17 which has been actuated at the third speed is also actuated, and the fourth speed is accomplished thereby.
When the shift select lever is at the "R" range, the hydraulic pressure passing through the manual valve 7 is supplied to the right side of the second-third speed and fourth-third speed shift valve 11 via the rear clutch exhaust valve 12 to push the valve spool leftward, and thus the hydraulic pressure passing through the manual valve 7 is supplied to the front clutch 13 and the low-reverse brake 15 and acts to deactivate the kick down servo brake 16 to drive the vehicle in the reverse direction.
As described hitherto, the fluid supply efficiency of the above-mentioned automatic transmission control system is bad because the shifting is performed simply between the first speed and the fourth speed and the hydraulic pressure from the hydraulic pump is supplied in two modes only. Also it is difficult to improve the pump efficiency and the shifting effect because the shock in the N-D control valve is controlled by the duty-control of the pressure control valve when N-D shifting. Furthermore, it is not possible to perform a skip shifting, which results in a slow response, because the individual clutches are not independently controlled, and it has a complicated construction because the shift valves are controlled indirectly.